Final answer:
A medium designed to change color for bacterial growth indication is a differential medium. It enables differentiation between microbial colonies based on color changes due to enzymatic reactions or other biochemical processes.
Step-by-step explanation:
A medium that is designed to change color to indicate the growth of certain types of bacteria would be considered a differential medium. This type of medium makes it easy to distinguish colonies of different microbes by the change in color, either of the colonies or of the medium itself, owing to interactions between bacterial enzymes and substrates or other reactions such as hemolysis.
For example, MacConkey agar is a differential medium that allows the observation of lactose fermentation. Lactose fermenters produce acid which leads to a hot pink color in the colonies or the medium due to the pH indicator present, such as neutral red that turns pink at low pH levels.
Selective media, on the other hand, selectively encourage the growth of certain bacteria while inhibiting others, often using antibiotics or specific growth inhibitors like bile salts. Enriched media contain additional nutrients essential for the growth of fastidious organisms. Chemically defined media are composed of known quantities of all their chemicals, whereas complex media contain ingredients like yeast extracts where exact chemical compositions are not fully known.